Chapter 4 Section 2 The Visual System - PowerPoint Presentation

Slide1

Chapter 4 Section 2

The Visual System

Goal Two: Explain how the visual system enables us to see, and by communicating with the brain, to perceive the world.Slide2

The Visual Stimulus and the Eye

Light

is a form of electromagnetic energy. Light travels through space in waves

.

The

wavelength

of light is the distance from the peak of one wave to the peak of the next

.

Amplitude

is the height of the wave and it is associated with the brightness of a visual stimulus

.

Purity

is the mixture of wavelengths in light.Slide3
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The Structure of the Eye

The eye is set up like a camera in that it gets the picture of the world

.

The

sclera

is the white outer part of the eye that gives the eye its shape and protects the eye from injury

.

The

iris

is the colored part of the eye

.

The

pupil

, which appears black, is the opening in the center of the iris. The iris contains muscles that allow the pupil to get larger or smaller depending on how much light is being let into the eye.Slide5

The Structure of the Eye

The

cornea

is the clear membrane on the outer part of the eye. The curved surface on the cornea bends light on the surface of the eye in order to focus it to the back of the eye

.

The

lens

is transparent and somewhat flexible. When a person is looking at an object far away, the lens has a relatively flat shape. However, when a person is looking at an object that is closer, more bending of the light is needed.Slide6

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Structure of the EyeSlide7

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Structure of the Eye: Retina

Rods

sensitive to even dim light, but not color

function well in low illumination

humans have ≈ 120 million rods

Cones

respond to color

operate best under high illumination

humans have ≈ 6 million conesSlide8
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Structure of the Eye: Retina

Fovea

densely populated with cones

vital to many visual tasks

Bipolar and Ganglion Cells

Optic Nerve

Blind Spot

where optic nerve leaves the eyeball

perception involves top-down processingSlide10

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Structure of the Eye: RetinaSlide11

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Visual Processing

Pathway of Visual Information

optic nerve

 o

ptic chiasm

 …  v

isual cortex

Optic Chiasm: Optic Nerve Fibers Divide

left visual field

 right hemisphere

right visual field

 left hemisphere

Primary Visual Cortex

occipital lobe

initial visual processingSlide12

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Visual ProcessingSlide13

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Visual Processing

Feature Detectors

highly-specialized cells in the visual cortex

size, shape, color, movement, or combination

deprivation studies: brain “learns” perception

Parallel Processing

BindingSlide14

Parallel Processing

What” and “where” are two questions that need to be answered in order for people to respond appropriately to a visual stimulus

.

The “what” pathway is in the temporal lobe and processes information about what the object is. The “where” pathway is located in the parietal lobe and processes information about an object’s location

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Parallel processing

is a simultaneous distribution of information across different neural pathways. It helps information move rapidly through the brain.Slide15

Binding

Binding

is when the different pathways and cells bring together and integrate information

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Through binding a person can integrate information about various parts of an object. For example, if a person sees a chair, then through binding they not only see just the chair, but they also see the size, the color, the motion, etc. of the chair.Slide16

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Color Vision: Theories

Trichromatic Theory

Three Types of Receptors

green

,

red

,

and

blue

cones

Color Blindness

one or more cone types is inoperativeSlide17

Color Vision: Theories

The term

color blind

refers to seeing some colors but not others. Color blindness depends on which of three kinds of cones (green, red, and blue) is not working.

The

opponent-process theory

states that cells in the visual system respond to red-green and blue-yellow colors. A cell excited by red and green could be inhibited, or a cell excited by blue and the yellow could be inhibited.Slide18

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Color Vision: AfterimagesSlide19

Color Vision: Theories

Afterimage

sensation remains after a stimulus is removed

trichromatic theory cannot explain afterimages

Opponent Process Theory

complementary color pairs

© 2011 The McGraw-Hill Companies, Inc.Slide20

Perceiving Shape, Depth, Motion, and Constancy

To perceive a visual stimulus, the fragments of information that the eye sends to the visual cortex must be organized and interpreted

.

The

figure-ground relationship

occurs when a person organizes the perceptual field into stimuli that stand out (figure) and those that are left over (ground

).

Gestalt psychology

explains how people naturally organize their perceptions according to certain patterns. Closure is a gestalt principle; when a person sees a disconnected or complete figure, they see a whole. Proximity is a second gestalt principle; when individuals see objects as close to each other, they tend to group them as together. The third gestalt principle is similarity; when objects are similar, individuals tend to group them together.Slide21

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Visual Perception

organizing and interpreting visual signals

dimensions

- shape

- depth

- motion

- constancySlide22

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Visual Perception: Shape

Gestalt Psychology

perceptions are naturally organized according to certain patterns

whole is different from the sum of the parts

Gestalt Principles

figure-ground relationship

closure

proximity

similaritySlide23

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Figure-Ground RelationshipSlide24

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Visual Perception: ShapeSlide25

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Visual Perception: Depth

the brain constructs perception of 3D from 2D images processed by the retina

binocular cues

- disparity

- convergenceSlide26

Depth Perception

Depth perception

is the ability to see objects in three dimensions

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Binocular cues

are depth cues that depend on the combination of the images in the left and right eyes and on the way the two eyes work together. For example, if a person holds their hand over one eye and focuses on an object and then switches to cover their other eye, the switching back and forth between the eyes will cause the object to jump back and forth

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Monocular cues are depth cues that are available from the image in one eye, either the left or the right eye. Some examples of monocular cues are: familiar size, height in the field of view, linear perspective, overlap, shading, and texture gradient.Slide27

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Visual Perception: Depth

Monocular Cues – Pictorial Cues

familiar size

height in the field of view

linear perspective

overlap

shading

texture gradientsSlide28

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Monocular Cues:

Shading, Texture GradientSlide29

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Monocular Cues:

Linear Perspective, Height in FieldSlide30

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Visual Perception: Motion

humans have specialized motion detectors

apparent movementSlide31

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Visual Perception: MotionSlide32

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Visual Perception: Constancy

Perceptual Constancies

recognition that objects do not physically change despite changes in vantage point and viewing conditions

sensory information (retinal image) changes, but perceptual interpretation does not

Size, Shape, and Color ConstanciesSlide33

Motion Perception

Apparent motion

occurs when an object is stationary but it is perceived as moving. An example of apparent motion is watching an IMAX movie. Two forms of apparent motion are stroboscopic motion and movement aftereffects

.

Perceptual constancy

refers to the recognition of objects as remaining stationary and unchanging even though sensory input about them is changing. There are three types of perceptual constancy: size constancy, shape constancy, and brightness constancy.